The concentration of crosslinks in gels or rubbers is closely related to their physical properties. Patent Literatures 1 to 3 propose methods for evaluating the concentration of crosslinks in a crosslinked rubber using a test to indicate the crosslink concentration. In this test, a crosslinked rubber is swollen in toluene, which is a good solvent for the polymer in a crosslinked rubber, and the ratio of the volume of the rubber before and after swelling is used as the degree of swelling to evaluate the crosslink concentration.
Moreover, according to the disclosure of Non-Patent Literature 1, a crosslinked rubber swollen as described above is analyzed by small-angle neutron scattering to evaluate the size of network inhomogeneities which correspond to portions of the swollen crosslinked rubber that have a relatively high crosslink density.
The methods by measuring the degree of swelling as disclosed in Patent Literatures 1 to 3 merely allow for the evaluation of the overall concentration of crosslinks in a crosslinked rubber and cannot be used to determine details of the variation in crosslink concentration, e.g. the crosslink concentration in network inhomogeneities. Moreover, the methods by evaluating the size of network inhomogeneities by small-angle neutron scattering as disclosed in Non-Patent Literature 1 merely evaluate the size of network inhomogeneities in crosslinked rubber materials and cannot be used to determine the crosslink concentration in the network inhomogeneities. Thus, there is a need for a method for evaluating crosslink concentration which is closely related to the physical properties.